1. Field of the Invention
This invention relates to forehearth systems for conditioning molten glass from a glass melter and rendering it suitable for subsequent processing, such as forming it into a desired shape. More particularly, this invention relates to burner systems suitable for use in forehearth systems.
2. Description of Prior Art
The forehearth section of a glass manufacturing operation is disposed between the glass melting furnace in which the raw materials for making glass are melted and the processing section in which the molten glass is processed into the desired form or shape. The forehearth system is designed to receive molten glass from the glass melting furnace and convey it to the glass processing operation, conditioning the molten glass during conveyance between the glass melter and the glass processing operation, thereby rendering it suitable for processing. In particular, the forehearth is designed to heat or cool the glass to the temperature required for processing.
The typical forehearth system comprises a cooling section which receives molten glass from the melter portion of the furnace, and a front conditioning section which receives molten glass from the cooling section. The conditioning section lies between the cooling section and the glass processing section. The cooling and conditioning sections are provided with independently controlled firing systems.
The cooling section of the forehearth system receives molten glass from the melter and cools or heats it to the proper average temperature required for the type of glass being made, such as containers made by a forming machine or fibers stretched by various attenuation devices. When the desired glass temperature cannot be obtained by radiation alone while maintaining properly set flames above the molten glass, additional cooling air is introduced into the cooling section of the forehearth above the molten glass.
From the cooling section, the glass flows into the conditioning section of the forehearth in which the temperature of the glass is equalized only by heating, using burners disposed within the walls of the forehearth, and not by cooling. The temperature in the conditioning section is controlled independently of the temperature in the cooling section. The conditioning section is intended only to hold and equalize the temperature and, thus, the viscosity of the glass.
Traditional firing systems for heating the glass in the conditioning section of a forehearth system are of a combustion premix design where the fuel, for example natural gas, and the combustion air are premixed together before they are introduced to the burner. See for example, U.S. Pat. No. 5,169,424 which generally teaches a forehearth structure for a glass melting furnace including gas burners for providing heat to the molten glass flowing through the forehearth. See also U.S. Pat. No. 4,662,927 which teaches a forehearth system having fuel-air burner nozzles which provide a flame for heating the space above the flowing molten glass and U.S. Pat. No. 4,708,728 which teaches a premixed fuel-air burner for heating the forehearth of a glass melter, the burner having a capillary tube disposed coaxially therein and extending beyond the end of the burner for feeding oxygen into the fuel-air premixture.
However, numerous problems are associated with traditional forehearth firing systems which employ premixed air-fuel burners for heating the flowing glass including poor fuel efficiency, little or no flame luminosity, very limited turndown ratio, a high volume of combustion gases and associated emissions within and outside of the glass plant, a generally high noise level due to the air-gas combustion system and, finally, the inability to provide precise temperature control of the glass, as small as 1.degree. or 2.degree. F., due to the significant variations in atmospheric air used by air-gas firing systems.
In traditional air-gas fired forehearth systems, the individual burner firing capacity, placement of burners, cooling air flow rate and location, and glass level within the forehearth system are controlled to achieve the objectives of glass temperature uniformity and glass viscosity in accordance with the requirements of the particular glass processing operation. Other than changing the firing rate of individual burners, alteration of each of these control parameters results in increases in capital costs, operating costs, and/or interruption of the glass manufacturing process.
The most common approach to controlling the temperature of a given zone within a forehearth system is to adjust the firing rate of the burner(s) corresponding to the particular zone to compensate for the net heat loss or heat gain within the zone. Known temperature control systems consist essentially of thermocouples immersed in the molten glass or a radiation pyrometer sighted on the surface of the glass, an electronic-type recording instrument for recording and controlling the temperature of the glass, and an electrically operated valve for adjusting the fuel input.
U.S. Pat. No. 3,321,288 teaches a temperature control apparatus for controlling the temperature of glass entering the forehearth of a glass melter in which cooling air and hot combustion gases are continuously supplied over the molten glass in the temperature control zone and the volumes thereof are varied relative to one another to maintain the temperature of the glass downstream of the zone substantially constant.
As previously stated, one of the disadvantages of a premixed air-fuel firing system is the very limited turndown ratio which, in turn, limits the level of control on the forehearth when responding to a temperature control signal to either decrease or increase fuel input. Turndown ratio, that is, the high firing rate of the burner divided by the low firing rate of the burner, for a premix burner is about 4:1 because velocities of the premixed air-gas flame which are too low result in flashback while velocities which are too high will blow the flame from the burner nozzle. To improve the turndown ratio of forhearth burners, it is known to change the premix burner nozzle design based on the type of flame sought to be produced. However, burner performance is only generally marginally improved by implementation of these techniques with respect to turndown.
Another disadvantage of premix air-gas flames is the fact that they are generally very short and transparent. Flame luminosity is almost nonexistent. A short, nonluminous flame is considered very poor for imparting radiative heat flux to the molten glass because the wave length for improved transmittance through the majority of glass falls between about 0.5 to 2 micron meters. Thus, the flame should have visible radiation wave length as emitted by the combustion of soot particles. The absence of a luminous flame results in a very non-efficient heating process, the combustion gases failing to efficiently release heat to the glass and forehearth superstructure. Thus, this heat is generally carried away in the form of exhaust gases to the stack, resulting in a net heat loss.
Finally, compared to oxygen-utilizing combustion systems, the fuel efficiency of an air-gas system is significantly inferior due to nitrogen contained in the air. It is known that, for example, 100% oxygen-gas combustion can reduce fuel consumption by about 60% compared to air-gas combustion without any heat recovery. U.S. Pat. No. 5,147,438 teaches an auxiliary oxy-fuel burner for glass melting having a central fuel nozzle disclosed concentrically within an oxygen nozzle; U.S. Pat. No. 4,531,960 teaches a process for making glass using air-fuel and oxygen-fuel burners where the flame produced by the oxygen-fuel burners is surrounded by a current of auxiliary gas, such as air or nitrogen, introduced through an annular space surrounding the burner; and U.S. Pat. No. 5,092,760 teaches an oxy-liquid fuel burner where oxygen or carbon dioxide are used as an atomizing fluid for the liquid fuel.
However, none of the prior art of which we are aware teaches or suggests an oxy-fuel fired burner for use in the forehearth system of a glass melting furnace as a way of addressing the problems associated with premixed air-fuel firing as discussed hereinabove.